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Rollup merge of rust-lang#103346 - HeroicKatora:metadata_of_const_poi…
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…nter_argument, r=dtolnay

Adjust argument type for mutable with_metadata_of (rust-lang#75091)

The method takes two pointer arguments: one `self` supplying the pointer value, and a second pointer supplying the metadata.

The new parameter type more clearly reflects the actual requirements. The provenance of the metadata parameter is disregarded completely. Using a mutable pointer in the call site can be coerced to a const pointer while the reverse is not true.

In some cases, the current parameter type can thus lead to a very slightly confusing additional cast. [Example](HeroicKatora/static-alloc@cad9377).

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = val as *const _ as *mut T;
let ptr = uninit.as_ptr().with_metadata_of(ptr);
```

This could then instead be simplified to:

```rust
// Manually taking an unsized object from a `ManuallyDrop` into another allocation.
let val: &core::mem::ManuallyDrop<T> = …;

let ptr = uninit.as_ptr().with_metadata_of(&**val);
```

Tracking issue: rust-lang#75091

``@dtolnay`` you're reviewed rust-lang#95249, would you mind chiming in?
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Dylan-DPC authored Oct 22, 2022
2 parents 3f49f95 + e3606b2 commit b22559f
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Showing 3 changed files with 7 additions and 3 deletions.
2 changes: 1 addition & 1 deletion library/alloc/src/rc.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1386,7 +1386,7 @@ impl<T: ?Sized> Rc<T> {
Self::allocate_for_layout(
Layout::for_value(&*ptr),
|layout| Global.allocate(layout),
|mem| mem.with_metadata_of(ptr as *mut RcBox<T>),
|mem| mem.with_metadata_of(ptr as *const RcBox<T>),
)
}
}
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2 changes: 1 addition & 1 deletion library/alloc/src/sync.rs
Original file line number Diff line number Diff line change
Expand Up @@ -1204,7 +1204,7 @@ impl<T: ?Sized> Arc<T> {
Self::allocate_for_layout(
Layout::for_value(&*ptr),
|layout| Global.allocate(layout),
|mem| mem.with_metadata_of(ptr as *mut ArcInner<T>),
|mem| mem.with_metadata_of(ptr as *const ArcInner<T>),
)
}
}
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6 changes: 5 additions & 1 deletion library/core/src/ptr/mut_ptr.rs
Original file line number Diff line number Diff line change
Expand Up @@ -80,10 +80,14 @@ impl<T: ?Sized> *mut T {
#[unstable(feature = "set_ptr_value", issue = "75091")]
#[must_use = "returns a new pointer rather than modifying its argument"]
#[inline]
pub fn with_metadata_of<U>(self, mut val: *mut U) -> *mut U
pub fn with_metadata_of<U>(self, val: *const U) -> *mut U
where
U: ?Sized,
{
// Prepare in the type system that we will replace the pointer value with a mutable
// pointer, taking the mutable provenance from the `self` pointer.
let mut val = val as *mut U;
// Pointer to the pointer value within the value.
let target = &mut val as *mut *mut U as *mut *mut u8;
// SAFETY: In case of a thin pointer, this operations is identical
// to a simple assignment. In case of a fat pointer, with the current
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